1. Field of the Invention
This invention relates generally to nickel-hydrogen batteries and more particularly to a nickel-hydrogen battery having a bipolar structure.
2. Description of the Prior Art
A typical prior art nickel-hydrogen battery comprises a hydrogen gas filled pressure vessel in which a single or a pair of cells are located. Each cell has a positive nickel electrode spaced apart from a negative hydrogen electrode. A separator is located between the nickel and hydrogen electrodes. Each separator is sufficiently thick to prevent electrical shorting between the electrodes. In addition, each separator holds electrolyte therein to allow an electro-chemical reaction to occur in the cell. Generally, the electrolyte is an aqueous alkaline solution, preferably a solution of potassium hydroxide.
During the normal charging cycle and during over-charging, oxygen gas is produced in the cell by the electrolysis of water in the electrolyte. This oxygen must be recombined with the pressure vessel hydrogen gas to form water within the cell to prevent the build up of oxygen gas pressure and a drying out of the separator. A build up of oxygen gas pressure may cause a breach in the pressure vessel structure. Separator dryout can lead to reduction in cell performance and a shortened cell lifetime.
Typically, pairs of cells are arranged with the nickel electrodes in a back to back configuration. This is illustrated in U.S. Pat. No. 4,115,673, issued to Van Ommering et al, and U.S. Pat. No. 4,127,703, issued to Holleck. The Van Ommering et al patent describes a stack of cells wherein a cell pair, or module, is separated from an adjacent module by a module separator. The module separator prevents electrolyte contact between adjacent modules and the cells contained within the adjacent modules. The Holleck patent also describes a back to back arrangement of cell pairs. The Holleck patent includes an electrolyte reservoir and a microporous hydrophobic membrane beteween the back to back electrodes. The membrane permits gas and vapor to flow therethrough while being impermeable to the liquid electrolyte. In the back to back cell arrangement, there is an exchange or sharing of water between the cells during the charging and discharging of the cells. This exchange or sharing of water occurs through the electrolysis and oxygen-hydrogen recombination phases.
The Holleck patent further describes a cell stack wherein a single positive configuration is used. In this configuration there is transport of oxygen gas evolved in one cell to an adjacent cell where it is recombined. This arrangement leads to oxygen gas transport in a single direction. Holleck provides for returning the oxygen at one end through a return conduit to the other end for oxygen-hydrogen recombination. This oxygen return scheme prevents an asymmetric buildup of water in the cell stack. Thus, this single positive configuration also results in the exchange of water between cells.
U.S. Pat. No. 3,975,210, issued to Warnock, also describes a plurality of paired cells within a single pressure vessel. To accommodate the paired cells in the pressure vessel, each cell pair is contained with an individual compartment or module. The modules are constructed to permit hydrogen gas flow into the module but restricts electrolyte from leaving. Therefore, the loss of electrolyte and water from the module is minimized. However, the cell pair shares water in the module as does the previously described prior art patents.
In each of the previously described prior art patents, the nickel electrodes are electrically connected in parallel. In addition, the hydrogen electrodes are also electrically connected in parallel. Each electrode has a tab which is then connected to a busbar for the respective nickel or hydrogen electrodes. These tabs and busbars have the inherent disadvantage of adding additional weight to the battery structure.
A nickel-hydrogen battery having a bipolar cell configuration alleviates the need for electrode connecting tabs and busbars. By eliminating electrode tabs and busbars, significant weight savings can be obtained in a large system.
In a bipolar cell configuration, a plurality of individual cells are stacked such that a first cell's nickel electrode is in electrical contact with an adjacent second cell's hydrogen electrode. Thus, current is permitted flows axially through the cell stack during the charging and discharging of the battery.
In a bipolar cell configuration, electrolyte must be contained within each individual cell to prevent electrolyte bridging between cells. Electrolyte bridging would result in parasitic shunt currents, thus limiting the performance of the battery. Therefore, in a bipolar battery, careful water management is required to prevent electrolyte bridging and separator dry out. This requires that oxygen gas generated by electrolysis of the water in the electrolyte during charging and overcharging, to be combined within the same cell. However, no proper solution for water control on an individual cell basis has been demonstrated in the prior art.